The above negative flow control system refers to a system in which when a pilot signal pressure generated from a pilot signal pressure-generating means installed at the downstream side of a center bypass path is high at the upstream side of the center bypass path, a discharge flow rate of a variable displacement hydraulic pump is controlled to be decreased whereas when the pilot signal pressure generated from a pilot signal pressure-generating means is low at the upstream side of the center bypass path, the discharge flow rate of the variable displacement hydraulic pump is controlled to be increased.
A conventional hydraulic circuit for a pipe layer in accordance with the prior art as shown in FIG. 1 includes:
first and second variable displacement hydraulic pumps (hereinafter, referred to as “first and second hydraulic pumps”) P1 and P2 and a pilot pump P3, which are configured to be connected to an engine 1;
one or more first control valves 3, 4 and 5 installed in a center bypass path (cbp) 2 of the first hydraulic pump P1 and configured to be shifted to control a flow direction and a flow rate of a hydraulic fluid that is supplied to a left traveling motor and a first work apparatus (or a swing motor, a winch motor, or the like);
one or more second control valves 7 and 8 installed in a center bypass path 6 of the second hydraulic pump P2 and configured to be shifted to control a flow direction and a flow rate of a hydraulic fluid that is supplied to a right traveling motor and a second work apparatus (or a boom cylinder or the like);
a straight traveling valve 9 installed at the upstream side of the center bypass path 6 of the second hydraulic pump P2, and configured to be shifted by a pilot signal pressure Pi from the pilot pump P3 to cause the hydraulic fluid discharged from the first hydraulic pump P1 to be distributed and supplied to the control valves 3 and 7 for the left and right traveling motors and to cause the hydraulic fluid discharged from the second hydraulic pump P2 to be distributed and supplied to the control valves 4, 5 and 8 for the first and second work apparatuses to thereby prevent one-way traveling when a combined operation mode for simultaneously driving the work apparatus and a traveling apparatus is selected;
an unloading valve 10 configured to be shifted by the pilot signal pressure that shifts the straight traveling valve 9 so that when the unloading valve is opened, the straight traveling valve 9 is shifted to prevent an overload from occurring in the center bypass paths 2 and 6 of the first and second hydraulic pumps P1 and P2;
one or more pilot valves 10 and 11 configured to release an unloading function of the unloading valve 10 when any one of the control valves 4, 5 and 8 for the work apparatuses and the control valves 3 and 7 for the traveling motors is driven in a shift mode in which the straight traveling valve 9 is shifted;
an operation mode switching valve 13 configured to be shifted in response to an electrical signal applied thereto from the outside when a combined operation mode for simultaneously driving the work apparatus and the traveling apparatus is selected to cause the pilot signal pressure from the pilot pump P3 to be supplied to the straight traveling valve 9 and the pilot valves 11 and 12, respectively; and
a first shuttle valve 14 configured to control a swivel angle of a swash plate (a) of the first hydraulic pump P1 by a pressure selected from among a pilot signal pressure Pi1 supplied to the pilot valve 12 and a pressure at the downstream side of the center bypass path 2 of the first hydraulic pump P1, and a second shuttle valve 15 configured to control a swivel angle of a swash plate (b) of the second hydraulic pump P2 by a pressure selected from among a pilot signal pressure Pi2 supplied to the pilot valve 12 and a pressure at the downstream side of the center bypass path 6 of the second hydraulic pump P2.
In the drawings, a non-explained reference numeral 24 denotes cbp spools respectively installed at downstream sides of the center bypass paths 2 and 6, and a non-explained reference numeral 16 denotes a main control valve (MCV).
The operation of a hydraulic circuit for a pipe layer to which the negative flow control system as constructed above will be described hereinafter with reference to the accompanying drawings.
The hydraulic fluids discharged from the first hydraulic pump P1 and the second hydraulic pump P2 are dividedly supplied to the main control valve (MCV) 16 and the unloading valve 10 via the center bypass paths 2 and 6, respectively. The unloading valve 10 is not used in an excavation operation mode of the equipment, but is used when a pipe-laying operation (PL) mode signal is activated.
In the pipe-laying operation mode, when the operation mode switching valve 13 is shifted, the straight traveling valve 9 is shifted to a state shown in FIG. 1 by the pilot signal pressure supplied to a port Ts (referring to a signal pressure port formed at the main control valve 16 to shift the straight traveling valve 9) from the pilot pump P3
As a result, a part of the hydraulic fluid discharged from the first hydraulic pump P1 is supplied to the control valve 3 via the center bypass path 2 to drive the left traveling motor. At the same time, a part of the hydraulic fluid discharged from the first hydraulic pump P1 is supplied to the control valve 7 through the shifted straight traveling valve 9 via the center bypass path 2 and a flow path 25 to drive the right traveling motor.
On the other hand, a part of the hydraulic fluid discharged from the second hydraulic pump P2 is supplied to the control valves 4 and 5 via the center bypass path 6, the straight traveling valve 9, and the flow path 26 to drive the first work apparatus (or a swing motor or the like). At the same time, a part of the hydraulic fluid discharged from the second hydraulic pump P2 is supplied to the control valve 8 via the center bypass path 6 and the flow path 27 to drive the second work apparatus (or a boom cylinder or the like).
As described above, when the operation mode switching valve 13 manipulated by an operator during the pipe-laying operation, the straight traveling valve 9 is shifted by the pilot signal pressure supplied from the pilot pump P3 to cause the hydraulic fluid discharged from the first hydraulic pump P1 to be distributed and supplied to the left and right traveling motors and the hydraulic fluid discharged from the second hydraulic pump P2 to be distributed and supplied to the work apparatus (or a boom cylinder or the like).
Therefore, in the pipe-laying operation mode, when the work apparatus and the traveling apparatus are driven simultaneously, the traveling speed can be prevented from being changed abruptly due to a difference in a load occurring in the work apparatus or the traveling apparatus
In the meantime, a signal pressure (40 kg/cm2) is applied to the unloading valve 10 from the pilot valve 12 to open the unloading valve 10 by the signal pressure supplied to the pilot valve 12 through a signal line 17 connected to the port Ts. At the same time, the signal pressures of the outlet ports A1 and A2 of the pilot valve 12 are supplied to the ports Pi1 and Pi2 of the via the signal lines 18 and 19 after passing through the first and second shuttle valves 14 and 15 installed at the downstream side of the pilot valve 12, respectively. As a result, the swivel angles of the swash plates (a and b) of the first and second hydraulic pumps P1 and P2 is controlled by the regulators R1 and R2 to minimize the discharge flow rate of the first and second hydraulic pumps P1 and P2.
In addition, the hydraulic fluid of signal lines 20 and 21 discharged from the main control valve 16 is set to be introduced into the first and second shuttle valves 14 and 15 to minimize the discharge flow rate of the first and second hydraulic pumps P1 and P2.
This state is defined as a neutral state of the pipe-laying operation mode.
In this case, in the neutral state of the pipe-laying operation mode, when signals (i.e., a manipulation signal by an attachment control joystick and a manipulation signal by a travel control pedal) of attachment switching devices (for example, a hoist winch (HW), a swing (SW), a boom (BM) and a circuit in which the ports PS1 and PS2 are indicated) 30 and 40 is activated, the pilot valve 12 is shifted with Pi1 by the hydraulic fluid (having a pressure of 40 k/cm2 or so) applied at the port PS2 (or PS1) of the attachment switching device 40. At the same time, the valve spools (or cbp spools) 24 of the main control valve 16 are shifted through the signal line 20.
When the valve spools 24 are shifted, respectively, the hydraulic fluid introduced into the main control valve 16 from the first hydraulic pump P1 and supplied to the hydraulic tank T, and the hydraulic fluid introduced into the main control valve 16 from the second hydraulic pump P2 and supplied to the hydraulic tank T are blocked, respectively.
When the pilot valve 12 is shifted, the hydraulic fluid of the port Ts is blocked at the pilot valve 12, and the hydraulic fluid of the port Pil disappears while flowing along a tank line 22 from the port A1 by the shifted pilot valve 12. In this case, the pressure applied to the first shuttle valve 14 at the downstream side of the port A1 also disappears simultaneously. As a result, when the pressure of the signal line 19 is reduced to cause the pressure of the port Pil of the first hydraulic pump P1 to be reduced to maximally control the discharge flow rate of the first hydraulic pump P1. At the same time, when the valve spools 24 of the main control valve 16 are shifted, the hydraulic fluid of a signal line 23 of the main control valve 16 is blocked and thus the pressure of the port Pil of the first hydraulic pump P1 is reduced via the first shuttle valve 14 to maximally control the discharge flow rate of the first hydraulic pump P1. At this time, the hydraulic fluid flowing to the hydraulic tank T from the port P1 of the unloading valve 10 is blocked.
On the other hand, when a signal of the attachment switching device (for example, BM or SW) 30 is activated, the attachment switching device 30 is connected to the Pi2 of the pilot valve 12 to shift the pilot valve 12 to the left on the drawing sheet. At this same time, the pressure of the port A2 of the pilot valve 12 and the pressure of the port Pil of the pilot valve 11 nearly disappear. The port A1 of the pilot valve 11 and the port Pil of the unloading valve 10 are connected to the tank line 22, and thus the pressures of the port A1 of the pilot valve 11 and the port Pil of the unloading valve 10 disappear. In this case, the ports P2 and T of the unloading valve 10 are blocked. At the same time, the pressure of the port A2 of the pilot valve 12 disappears, and thus the pressure of the signal line disappears so that the discharge flow rate of the second hydraulic pump P2 is controlled to be discharged maximally. At this time, the maximally discharged hydraulic fluid is supplied to each attachment switching device.
In the meantime, the unloading valve 10 of a poppet type controls the flow rate of the hydraulic fluid in an ON/OFF manner by the pilot signal pressure applied from the outside. In other words, even if the pilot signal pressure of 1-40 kg/cm2 is supplied to the ports Pi1 and Pi2 of the unloading valve 10, the flow rate is controlled in the ON/OFF manner. Therefore, when the unloading valve 10 is closed, a cross-sectional area of the closed aperture of a flow path is abruptly reduced to bring about a hydraulic shock (see FIG. 2(a)). As a result, it can be found that even if a low pilot signal pressure is applied to the unloading valve 10, the flow rate of the hydraulic fluid discharged from the first hydraulic pump P1 and the second hydraulic pump P2 is suddenly increased (see FIG. 2(b)).
As described above, when the attachment is finely manipulated by a pilot check type unloading system in the pipe-laying operation mode, the center bypass path is blocked by the poppet closing of the unloading valve. For this reason, the conventional the hydraulic circuit for a pipe layer entails a problem in that the discharge flow rate of the hydraulic pumps is controlled to the maximum in terms of the characteristics of the negative flow control system to cause the pressure to rise due to the excessive flow rate of the hydraulic fluid discharged from the hydraulic pump, leading to generation of chattering.